The present invention relates to a method and apparatus for processing data signals, particularly to a method and apparatus for changing code information having M-bit data signals such as acoustic signals or image signals or the like, to code information having N bits wherein "M&gt;N".
When the code information of M bits is changed into that of N bits, there are several data signal processing methods used in the conventional processing means as follows:
i) Method of simply cutting off (M-N) bits of code information in the lower places (less significant bits; hereinafter referred to as lower bits) leaving the N-bits of code information in the upper places (more significant bits; hereinafter referred to as upper N bits) of the M-bit code information;
ii) Method of rounding off code information of the lower (M-N) bits, when the code information of the lower (M-N) bits is cut off leaving only the code information of the upper N bits in the original code information of M bits;
iii) So-called noise shaping method of obtaining N-bits of code information by adding the code information from the (M-N) bits which are cut off from the M-bits of code information and fed back, with code information of the next M bits;
iv) Method of improving characteristics in the so-called noise shaping method by providing a filter in the feedback loop; and
v) Method of improving conversion characteristics at a fine level by changing the discrimination threshold value in a temporally short period.
When the conventional processing means using the above-mentioned five methods convert M bits of code information to N bits wherein M&gt;N, there is the problem that data signals obtained by the N bits of code information largely decrease its quality in comparison with signals obtained by M bits of code information. This problem is illustrated with reference to FIGS. 1A-1F.
Curves S1-S3 in FIGS. 1A-1F show examples of an original analog signal waveform as an object for data signal processing, curves Ds1m, Ds2m and Ds3m show the condition wherein sampled values for sampling the original analog signals S1-S3 in a sampling period Ts are quantized to digital signals of M bits, and curves Ds1n, Ds2n and Ds3n correspond to curves including solid lines showing a quantized condition wherein the sampled values obtained by sampling the original analog signals S1-S3 with a sampling period Ts become digital signals of N bits which are 4 bits (M-N=4) smaller than the M bits. 1LSBm in FIGS. 1A-1F shows a quantization level value in units of one over 2 raised to the M-th power (2.sup.M) of a predetermined dynamic range of the digital signals' value, 1LSBn in FIGS. 1A-1F shows a quantization level value in unit of one over 2 raised to the N-th power (2.sup.N) of the predetermined dynamic range, and examples shown in these figures show a scenario wherein M-N=4.
As understood after referring to FIGS. 1A-1F, when code information of the original analog signals shown as the curves S1-S3 is quantized by M bits and is converted into N bits of code information wherein M&gt;N, since a large amount of code information is eliminated, the signal shape quality decreases.
Since both of the so-called noise shaping methods as examples of i) and ii) have noises distributed in a high frequency band, they present problems in that a dynamic range in the high band becomes narrower and a concentration of noise energy in the high band negatively influences the signal. Furthermore, even though conversion characteristics are improved in the conventional method v) for improving the conversion characteristics at a fine level by changing the discrimination threshold value in the time base with a short period, there is the problem that the S-N ratio deteriorates by increasing a noise level because of the addition of a dither.